The invention generally relates to a superconducting device having a rotor which is mounted so as to rotate about a rotation axis and contains a winding of conductors including superconductor material. Further, it may relate to a device including a refrigeration unit which is designed for a working gas and includes a cold head, which is arranged in the rotor and is thermally coupled to the winding for indirect cooling of the latter. The refrigeration unit can incude a stationary compressor unit located outside the rotor, as well as a transfer unit, arranged between the cold head and the compressor unit, having a sealing device for conveying the working gas between the stationary and rotating parts.
A superconducting device is disclosed by U.S. Pat. No. 5,482,919 A.
Besides metallic superconductor materials which have been known for a long time, e.g. NbTi or Nb3Sn, which have very low transition temperatures Tc and are therefore also referred to as low-Tc superconductor materials or LTS materials, metal oxide superconductor materials with transition temperatures above 77 K have been known since 1987. The latter materials are also referred to as high-Tc superconductor materials or HTS materials, and in principle make it possible to employ a cooling technique using liquid nitrogen (LN2).
Attempts are also being made to produce superconducting windings with conductors by using such HTS materials. It is found, however, that previously known conductors have only a comparatively low current-carrying capacity in magnetic fields with inductions in the tesla range. This frequently entails the requirement that, in spite of the inherently high transition temperatures of the materials which are used, the conductors of such windings nevertheless need to be kept at a temperature level below 77 K, for example between 10 and 50 K, so that significant currents can be carried with field strengths of a few tesla. Such a temperature level, on the one hand, is indeed significantly higher than 4.2 K, the boiling point of liquid helium (LHe), with which known metallic superconductor materials such as Nb3Sn are cooled. On the other hand, however, cooling with LN2 is uneconomical because of the high conductor losses. Other liquefied gases such as hydrogen, with a boiling point of 20.4 K, or neon, with a boiling point of 27.1 K, are ruled out because of their hazardous nature or for lack of availability.
For the cooling of windings having HTS conductors, in the temperature range, it is therefore preferable to use refrigeration units in the form of cryocoolers with a closed pressurised He gas circuit, for example of the Gifford-McMahon or Stirling type, or as a so-called pulse tube cooler. Such refrigeration units also have the advantage that the refrigeration power is available almost at the touch of a button, and the need for the user to handle low-temperature liquids is obviated. When such refrigeration units are used, a superconducting device, such as e.g. a magnet coil or a transformer winding, is cooled only indirectly by heat conduction to a cold head of a refrigerator (cf. e.g. xe2x80x9cProc. 16th Int. Cryog. Engng. Conf. (ICEC 16)xe2x80x9d, Kitakyushu, J. P., 20.-24.05.1996 pub. Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for the superconducting rotor, belonging to an electrical machine, which is disclosed by the US-A document cited in the introduction. The rotor contains a rotating winding of HTS conductors, which is to be kept at a desired operating temperature of between 30 and 40 K by way of a refrigeration unit designed as a Stirling or Gifford-McMahon or pulse tube cooler. To that end, in a particular embodiment, the refrigeration unit contains a co-rotating cold head (not explained further) whose colder side is thermally coupled to the winding indirectly via heat-conducting elements. The refrigeration unit of the known machine also contains a compressor unit, located outside its rotor, which delivers the required working gas to the cold head via a rotating coupling (not explained in detail) of a corresponding transfer unit. The coupling also supplies the necessary electrical energy, via two slip rings, to a valve mechanism of the refrigeration unit, which mechanism is integrated in the cold head. This concept entails the requirement that at least two gas connections need to be fed coaxially through the transfer unit and at least two electrical slip rings need to be provided. Accessibility of the co-rotating parts of the refrigeration unit and, in particular, of the valve mechanism in the rotor of the machine, is also impaired: it is therefore necessary to open the rotor housing when maintenance is required. The function of a conventional valve mechanism is also unreliable at fast rotation, as is the case with synchronous motors or generators.
It is an object of the present invention to develop a device which creates a reliable operation of the refrigeration unit, even at the rotational speeds, in a temperature range below 77 K with comparatively reduced equipment outlay.
This object is achieved by providing a superconducting device including a refrigeration unit. The superconducting device may include a rotor which is mounted so as to rotate about a rotation axis and a winding of conductors including superconductor material, as well as a refrigeration unit which is designed for a working gas and which includes at least one regenerative cryocooler. This cryocooler may have at least one cold head, which is arranged in the rotor and is thermally coupled to the winding for indirect cooling of the latter, and a stationary compressor unit located outside the rotor. The refrigeration unit also may include a transfer unit, arranged between the cold head and the compressor unit, having a sealing device and a gas coupling for conveying the working gas between the stationary and rotating parts, a direct connection for the working gas being provided between the gas coupling and the cold head.
In this context, the term xe2x80x9cdirect connection for the working gasxe2x80x9d is intended to mean that, after the gas transfer from the stationary part to the rotating part of the transfer unit (or in the reverse direction) no other parts which are absolutely necessary for the function of the refrigeration device, in particular electrical control lines and optionally a valve mechanism, need to be provided between the transfer unit and the cold head.
In the inventive configuration of the superconducting device, the compressor side (optionally together with a valve mechanism which may be required) of the refrigeration unit can be arranged stationary outside the rotor, while the rotating coupling lies between the compressor side (and the optional valve mechanism) with the electrical terminals, on the one hand, and the cold head proper, on the other hand. It has been discovered that such a situation is possible if a regenerative cryocooler is selected for the refrigeration unit. In this context, the term xe2x80x9cregenerative cryocoolerxe2x80x9d is intended to mean a cryocooler with a regenerator, or a regenerative working cycle, according to the standard classification of cryocoolers (cf. e.g. xe2x80x9cProc. 16th Int. Cryog. Engng. Conf. (ICEC 16)xe2x80x9d, Kitakyushu, J. P., 20.-24.05.1996, pub. Elsevier Science, 1997, pages 33 to 44). The number of gas connection lines leading to the cold head of such a cryocooler is then advantageously minimal.
Slip rings for electrical transmission may also be fully obviated. For instance, for a pulse tube cooler corresponding to said cryocooler type based on the principle with a second inlet, only a single gas feed to the rotor is necessary. Also, no mechanically moved parts are mounted on the rotor in this type, so that no maintenance work on the rotor is generally necessary either. The valve mechanism of the pulse tube cooler can be mounted at a suitable position outside the rotor, and is readily accessible for maintenance work.
Corresponding advantages are also obtained for the embodiment of a split Stirling cooler which makes do without a valve mechanism. It is hence preferable to select a pulse tube cooler or a split Stirling cooler as the regenerative cryocooler. This is because, with these cooler types, not only can said requirements concerning limited equipment outlay be readily satisfied; they are also to be regarded as sufficiently advanced with a view to use for rotating windings.
Advantageous refinements of the superconducting device according to the invention are given in the dependent claims.
For instance, the refrigeration unit may advantageously have a plurality of cold heads. Then, depending on the winding size, it is possible to achieve higher refrigeration powers. It is also possible to arrange these cold heads in such a way as to reduce possible imbalance problems of the rotor.
The cold head may, particularly advantageously, be designed in multiple stages. With its first stage, an electrical feed or a thermal radiation shield is then put at a comparatively higher intermediate temperature. With a correspondingly configured cold head, it is straightforward for different co-rotating parts to be kept respectively at a temperature level which is favorable for effective cooling.
It is also to be regarded as advantageous if the winding to be cooled, and therefore its superconducting material, is to be kept at a temperature below 77 K by way of the cold head and, in the case of using HTS material, preferably between 20 and 50 K. This is because, in this temperature range which can be maintained with comparatively limited refrigeration outlay, known HTS materials have a sufficient critical current density for customary applications.
It is also particularly advantageous if a vertical orientation of the rotation axis of the rotor is provided. With a corresponding orientation of the cold head, functionality can then be guaranteed even at high rotational speeds of the rotor, without entailing problems of space inside the rotor body relating to an extended cold head arrangement.